Star Tracer is a windows program that can be used to improve and enhance
"star-trail" photographs. Its two main functions are:

Filling in the "gaps" in star-trails that result when stacking multiple
images together to create a single star-trail image.

Extending the length of star-trails.

Star Tracer works by calculating the path of the stars, and then using
that information to either fill in the gaps and/or extend the length of
star-trails.
Conceptual Overview

Conceptually, there are
two main steps in using Star Tracer. The first main step is providing enough information
to Star Tracer so that it can determine the path of the stars. This is done
by "drawing" several "star lines" on the image. These lines tell Star Tracer
about the paths followed by individual stars in your image.
Star Tracer uses this information to "optimize" the project...determining the
trajectory of all the stars in the image. Even short star-trails (e.g. 60
seconds) contain enough information for Star Tracer to determine the trajectory that
the stars will take over the course of several hours.

The second main step is
creating the output image. Once the project has been
optimized, Star Tracer can "rotate" the image by any amount and any number of
times requested. You can choose to output the result as a single image,
a series of images or a multi-layer Photoshop file. In fact, the image
transformation is much more complex than a simple rotation, but for simplicity,
this document refers to this transformation as rotation.
Quick Start Tutorial

This section provides a very quick tutorial showing how to use Star Tracer. This is not
the only way to use Star Tracer but shows a general approach that should work
well under many circumstances.

Open a TIFF or JPEG image in Star Tracer using the File|Open menu item.
The image should have several visible star trails, either as a result of using a long
exposure or having stacked a number of shorter exposure images into a single composite.
The image should be displayed on Star Tracer's main screen.

Draw at least 20 "Star Lines" (the more the better) on the image using the
mouse. (Press the left button to start a line, drag the mouse and release the
button to complete the line.) Each star line should lie exactly on top of a
star trail.

Enter the field of view for your image in the Image
Properties Panel. If you need to calculate this value, you can do so using
an online calculator like this one.

Set the Lens Projection type in the Image
Properties Panel as appropriate for your lens. (If you don't know the answer,
it is very likely that the lens type is rectilinear or perhaps one of the two fisheye settings).

Press the Optimize Button. The optimizer will
estimate values for pitch and roll based on your inputs (star lines, lens type,
field of view).

Choose how much you want to rotate the image (in seconds), and how many
rotations to preform using the Star-trail
Settings Panel. If you aren't sure, try a test image using one rotation
and a small time amount (e.g. 5 minutes or 300 seconds). Your
artistic/creative goals determine the "correct" values at this stage.

The main screen of Star Tracer is where almost all interaction with the program
occurs. It allows you to load an image, tell the program about the image, optimize
the project and initiate the creation of an output file. The main screen contains
several regions (listed clockwise from top-left):

Each Star Tracer project contains one image, which is displayed in the image window.
The first step in creating any project is to add an image (using the File -> Open Image
menu item, or dragging-and-dropping an image onto the image window).
Star Tracer can handle JPEG and/or TIFF files. (TIFF files can be 8 or 16 bits per
channel with either 3 or 4 channels.)

After loading an image, you should draw several "star lines" on top of the image
using the mouse. These lines are what tell Star Tracer the direction that the
stars follow in your image. Click and hold the left mouse button to start a
line, move the mouse, and then release the button to end the line. The star line
is shown in red on top of the image. The start and end point of each star line should
lie on top of a star-trail in the image.

To delete a line, hold the keyboard ctrl
key down, and drag the mouse while pressing the left mouse button. A rectangular
region is drawn on the screen and when the left mouse button is released, and
star lines within that region are deleted.

After the project has been optimized, a number is displayed next to each
star lines. This number is a measure of "fit"...how close each star line drawn on the
image matches Star Tracer's estimate of the trajectory of the underlying star.
Lower values are better than higher values, and values under 1 are excellent. If
there are many lines with large values (i.e. 3 or higher), chances are good
that one or more of the star lines have been drawn incorrectly, the FOV is
specified incorrectly, and/or the optimizer has converged upon an incorrect solution.
The solution to any of these problems is to check that all star lines have been
drawn correctly, add more star lines, reset the image parameters and optimize
again.
Close-up Window

As you move the cursor around the image, a magnified version of the region
underneath the cursor is displayed in the close-up window. This can be
helpful for accurate placement of the star lines.
Image Properties Panel

The image properties panel shows the essential pieces of information needed
for any Star Tracer project. Together, these parameters are what Star Tracer uses to
determine the correct trajectory of the stars.

Field of view is the horizontal (i.e. left to right) field of view (FOV) of your image. Most images
from modern digital cameras contain enough information for Star Tracer to
be able to deduce the FOV. However, if not, then you can enter
it manually. It can be calculated quite easily using a field-of-view calculator
like the one on this page.
The optimizer can also deduce this value in many cases, although this is not always
reliable, and often requires the addition of many more star-lines in order to
generate a reasonable estimate. If possible, it is preferable to calculate and
enter this value rather than asking the optimizer to estimate it.

Distortion b. In general most lenses exhibit some kind of distortion (i.e.
deviation from a theoretically perfect lens). It is necessary to account for this
distortion so that Star Tracer can correctly compute the trajectory of the stars. In most
cases, the distortion can be adequately characterized using one parameter ("b"), and
the optimizer can deduce the value for that parameter based on the star-lines you create.
If you already know this value (e.g. you calculated it in another project), you
can enter the value, otherwise you can let the optimizer derive it.

Pitch and Roll. Together these two values tell Star Tracer about the relationship
between the camera position (and orientation) and the celestial sphere upon which the
stars appear to travel. The main goal of the optimizer is to deduce these values
based on the star-lines that you create.

Lens Projection. This tells Star Tracer about the type of lens used to create
the image. If you are uncertain of the lens projection, rectilinear will
almost certainly be the correct choice, as almost all camera lenses (excluding
fisheye lenses) use rectilinear projection. Star Tracer
also supports other projections that can be generated by panoramic stitching software
like PTAssembler

.

Next to all of text fields are checkboxes. If checked, the optimizer will attempt
to estimate appropriate values for these parameters. In general, all projects
will need to have pitch and roll optimized. Unless you are certain that your image has no
lens distortion (e.g. you have already corrected it using other software), then it is
usually beneficial to have the optimizer calculate a value for the "distortion b" parameter.

If you do not know the FOV of your lens, and cannot enter it manually, then you
can ask the optimizer to estimate FOV. Note, however, that this can be unreliable, and
the optimizer may sometimes calculate values that aren't correct. Adding more
star lines to the project may help the optimizer find a more reasonable value.

The reset button resets the image properties to their default values. This
can be useful if the optimized results aren't correct, and you need to change
some values and/or star-lines before optimizing again. Because the
initial parameter values influence the results calculated by the optimizer, it
is often beneficial to reset all values prior to optimizing.

The advanced button shows the advanced settings screen where you can choose
to enter and (configure optimization of) several other less frequently used
image properties.
Star Trail Settings Panel

This panel allows you to control the final output. You can configure the amount
by which to "rotate" the image, and the number of times to perform this rotation. Rotation
amounts are expressed in seconds, and control the distance by which stars
appear to move across the sky. Because the earth rotates 360 degrees in 24 hours
(86,400 seconds), one second corresponds to 0.00417 degrees of rotation.

The panel allows you to specify one or two different rotations simultaneously. If you only
wish to rotate in one direction then only one rotation need be specified. Note that
the entire image is rotated by Star Tracer, not just the stars. Note that other objects
(e.g. trees, buildings, etc.) will also be rotated, unless they are "masked",
so you may also want to add a mask to the image before rotation.

Regardless of how many rotations are specified on this panel, Star Tracer
always creates one "base" output image (or layer), containing the original image without
any rotation. Although not rotated, this base image (or layer) may differ from the original
image if Star Tracer is configured to correct
lens distortion and/or ignore masks when creating output.
Optimize and Create Buttons

The optimize button invokes the optimizer which generates estimates for the
selected image properties. Any properties that are checked in the image
properties panel and the advanced settings screen will be estimated by the
optimizer. In general, the more values that the optimizer tries to estimate,
the more star lines are needed to generate reliable estimates. Similarly, if
FOV is being estimated, it is recommended to add several additional star lines.

After optimization, Star Tracer analyzes the results and presents a message with
suggestions and recommendations if the results appear unreasonable. Incorrectly
placed star lines, incorrect specification of parameters (e.g. lens projection, FOV)
and an inadequate number of star lines can cause the optimizer to produce unreliable
results.

The create button starts the image creation process by displaying the output
screen where you can choose the format for the output image(s). Star Tracer
can either create a single "flattened" output file (in JPEG or TIFF format), a
multi-layer output file (in PSD or PSB format), or multiple output files (in TIFF
format). The flattened format files are composite images, created by layering
multiple rotated images on top of each other, choosing the brightest pixel at
each location, similar to Photoshop's "lighten" blending mode.

In all cases, regardless of the number of rotations, Star Tracer always creates one
"base" image (or layer) containing the original image without any rotation.
If configured on the options screen, Star Tracer can
ignore any masks when creating the base image.
Status and Information

Underneath the main image window are the status and information panels. These panels
show information about the project and display application messages
produced by Star Tracer during its operation.
Options Screen

Options Screen

The options screen contains the following choices:

Pause after optimizer completes. Specifies the
number of seconds to display the optimizer screen after it completes processing.

Number of simultaneous threads... Specifies the number
of threads to use when creating the output image. Threads process in parallel, so
using more threads can speed processing. However, more threads require more memory,
and if too many threads are used, Star Tracer may exhaust available memory. You can
also select "auto", and Star Tracer will decide by itself the appropriate number of threads.

Correct distortion... If checked, Star Tracer will
also correct (i.e. undo) the disotrion in the image when processing.

Apply input image mask... If checked, and if the
input image includes a mask, then that mask is applied to the output image as well.
Note that even if unchecked, pixels that are masked in the input image are excluded from rotation/processing.

The advanced settings screen contains additional parameters that describe the
image. In general, under most circumstances these can be left at their default value
of zero, and do not need to be optimized.

The "distortion a" and "distortion c" parameters are additional parameters
that can be used to characterize any lens distortion. Some lenses have unusual
lens distortions that may benefit from including either (or both) of these parameters
in the optimization.

The Horizontal and Vertical shift parameters are useful if a lens is not
centered correctly, or if the input image is a region cropped from a larger image. This
is often true if the input image has been produced by a panoramic stitching program
(like PTAssembler), but has been
cropped before being input to Star Tracer. These values are used to tell Star Tracer
the amount by which the center of the input image differs from the center of the
specified lens projection.
Output Screen

Output Screen

The output screen allows you to choose from the following output formats.

Single flattened JPEG file. Star Tracer produces a single image in JPEG format.
This image is a composite, created by blending all of the individual rotated images, using
the brightest value at each pixel location. This is similar to Photoshop's "Lighten"
blending mode.

Star Lines. How many star lines should be drawn on the image?
The number of star lines required varies based on a number of factors:

How long are the trails in the image? In general, longer star lines give the
optimizer more information and are less prone to minor placement errors. If the image
only has short trails, adding more lines can help the optimizer produce reliable results.

Is FOV known, or being estimated by the optimizer? The optimizer may not always produce reliable
estimates of FOV, but adding more star lines increases the chances of a good estimate.

How accurately are the lines placed? More lines can offset minor placement
errors in the position of the star lines.

How many parameters are being estimated by the optimizer? If you choose to optimize
some of the advanced lens parameters, then adding more lines can help produce
reliable results.

In general, it is recommended to create at least 10 lines per project. However,
depending on the factors described above, you may wish to add more. In some rare cases,
(e.g. unknown/optimized FOV, unusual lens distortion, optimized advanced parameters, short start-trails, etc. )
adding up to 100 lines or more may prove beneficial.

Masks. A "mask" is a region of the image that is excluded from processing
when Star Tracer rotates the image. For example, you may wish to mask all
"non-sky" regions of the image so that the resulting output image(s) only
contain the sky and stars. Star Tracer allows you to create one or more masks
when in "Mask Drawing Mode" (activated using the Tools menu). In this mode,
you can draw mask regions on the image by creating a collection of points to
define a mask region. Clicking on the image adds mask points to the image.
Double-clicking causes the mask region to close, i.e. it connects the first and last point
in the collection and "closes" the mask region. Right-clicking on the screen
brings up a context menu with additional choices to invert, delete and edit a mask region.
Right-clicking on existing mask point offers options to move and delete that point.

Star Tracer Masks

Star Tracer's mask creation capabilities are quite flexible, but in some
cases you may wish to take advantage of the controls offered by a full-featured
image editing program (like Photoshop) to create more complex masks, by adding a
fourth (sometimes called "alpha") channel to your image. Most images only
contain three channels (red, green and blue). However, if the image contains a
fourth channel, that channel is used as a mask by Star Tracer. Consult the
documentation for your prefered image editing program to learn how to create
a mask for your image. Note that TIFF files can store a fourth channel, but
not JPEG files.
Bugs and Problems

As hard as software developers try, most programs contain some bugs. If you find a
bug in Star Tracer please feel free to contact the author
(see Star Tracer's page for contact details).

When reporting a problem, please give as much information as possible. A
good bug report is one that allows the developer to reproduce the problem using
the steps that you provide. A step-by-step recipe that allows the developer to
replicate the problem is likely to result in a quick diagnosis and resolution.
Reports with vague wording such as "I saw an error message" or
"it doesn't work" make the diagnosis and resolution difficult or
impossible.

If reporting a bug, please explain what happened (what image format, what dimensions,
what were you trying to do, what buttons did you push, etc.). Please do not
send your image(s) unless the author asks for them. Most importantly, please
provide a step-by-step recipe that can be followed to elicit the problematic
behavior...if the problematic behavior can be replicated, it can usually be solved. If not,
a solution is often difficult or impossible.

More information is always better, and is more likely to lead to a solution...Thanks!

Star Tracer is free to download and use. However, until it has been registered,
the images that it generates contain a number of small watermarks. Use the registration
screen (the Help | Purchase/Register... menu option) to purchase
and/or enter a registration code. The license code for Star Tracer can be
purchased online (using the "Register online..." button), and that license code should
be entered into the registration screen's input fields.